Method of preparation of compressed vermicular graphite



United States Patent Oflice 3,448,181 Patented June 3, 1969 3,448,181METHOD OF PREPARATION OF COMPRESSED VERMICULAR GRAPHITE FranciszekOlstowski, Freeport, Tex., and Kenneth W.

Guebert, Midland, Mich., assignors to The Dow Chemical Company, Midland,Mich., a corporation of Delaware N Drawing. Filed Mar. 30, 1966, Ser.No. 538,589 Int. Cl. C01b 31/04 US. Cl. 264-120 6 Claims ABSTRACT OF THEDISCLOSURE This invention relates to a process for producing compressedgraphite structures from particulate vermicular graphite and moreparticularly relates to an improved process for producing relativelydense graphite structures from relatively low density vermiculargraphite requiring a minimum of volume change by mechanical compression.

It is known that expanded graphite in vermicular form is compressibleand masses of such vermicular graphite may be formed into shapedstructures approaching the theoretical density of the graphite. It islikewise known, however, that in compressing masses of vermiculargraphite such large volume changes are usually necessary that seriousmechanical difliculties are encountered, particularly in the formationof relatively high density structures.

It is an object of this invention to provide an improved process forproducing cohered graphite structures from vermicular graphite. Anotherobject is to provide a proc ess for compressing such vermicular graphitewherein the volume change during such compression is minimized. Theseand other objects and advantages of the present process will becomeapparent from a reading of the following detailed description.

It has now been discovered that relatively high density graphitestructures may be produced by first compressing the particulatevermicular graphite particles individually or in small groups, clustersor clumps into flattened flakes, then compressing the necessary quantityof such compressed graphite particles into the desired shaped structure.It has been found that coherence and interlocking between such flakes,when compressed as a random mass, is suflicient to produce a cohered,integral, monolithic structure having good structural strength. Bycompressing the individual particles or small clumps of particles, amass of flattened, irregularly-shaped, flakelike particles (hereinreferred to as compressed flakes) are produced whose bulk density iscomparatively high in relation to the vermicular form prior tocompression. Only a minimum volume change is required, therefore, tocompress a mass of such flattened particles into a relatively highdensity structure. A structure prepared from such compressed flakes isusually physically indistinguishable from one compressed directly fromvermicular graphite and has substantially the same electrical andthermal characteristics.

The vermicular graphite employed herein is a compressible form ofgraphite prepared by introducing an intercalating agent between thelaminae of natural or synthetic graphite and expanding such treatedgraphite by heating to a temperature above about 200 C., and usuallyabove 500 C. For example, a heat-expandable graphite may be prepared bycontacting graphite particles with an intercalating agent such as fumingnitric acid, fuming sulfuric acid, mixtures of concentrated nitric andsulfuric acids, perhaloacids or the like. The treated graphite particlesmay then be washed free of excess intercalating agent and dried ifdesired. The resulting treated graphite may be expanded in volume fromabout 20 to about 600 times by heat, e.g., with a propane flame. Suchexpanded vermicular graphite is usually in light weight, particulate,worm-like form and is easily malleable and compressible into shapedmonolithic structures.

Compression of such expanded vermicular graphite along a single axisproduces a compact integral structure having high electrical and thermalanisotropy. Both electrical and thermal resistivity are highest in thedirection of compression and lowest in the direction perpendicular tothat of compression. The anisotropy ratio of such compressed structuresincreases with increasing compression up to or near the theoreticaldensity of the graphite. Compression of vermicular graphite in twodirections substantially reduces the anisotropic properties of thecompressed structure and isostatic compression produces a structurehaving little or no anisotropy.

In the first step of this process, vermicular graphite particles orclumps of particles are compressed to form very thin, flattened, lacy,irregular, flake-like, separate particles having many times the diameterof the original particle or clump of particles and having high diameterto thickness ratios. Such compression may be accomplished by anyconventional means such as passing vermicular graphite particles betweenrollers, pressing between flat platens and the like but passing suchparticles between rollers is usually the most rapid and eflicient means.

Vermicular graphite, suitable for compression into strong unitarycompacts, has an apparent bulk density of between about 0.002 to about0.02 gm./cc. For use in the process of this invention, it is desirablethat such graphite be formed into compressed flakes having a particledensity in the range of from about 0.25 gm./cc. to about 1.5 gm./cc. andan apparent bulk density of from about 0.03 to about 0.2 gm./ cc. Thedensity of such compressed graphite flakes is easily controlled byvarying the force and, to some extent, the time of compression. Ifrollers are used to form such flakes from the vermicular graphite, thedensity of the compressed flake is easily controlled by controlling thegap between the rollers and their speed of rotation. A larger gapbetween the rollers and high roller speed produces lower density flakesWhereas a smaller roller gap and lower roller speed produces flakeshaving higher particle density. The density of such flakes may befurther increased by passage through more than one set of rollers or bymultiple passes through the same rollers.

Once the vermicular graphite has been compressed to form thin flexiblecompressed graphite flakes of the desired density, a random mass of suchflakes is compressed to form a shaped structure. Compression of a massof such flakes along a single axis will produce a compact or structuresimilar to one produced directly from the vermicular graphite, i.e., onehaving high electrical and thermal anisotropy. Both electrical andthermal resistivity are highest in the direction of compression andlowest in the direction perpendicular to that of compression. Theanisotropy ratio of such structure increases with increasing compressionup to or near the theoretical density of the graphite. Compression fromtwo directions substantial- 1y reduces the anisotropic properties of thecompressed structure and isostatic compression produces a structurehaving little or no anisotropy. By selective compression, therefore,shaped structures may be produced from compressed graphite flakes whichhave a density between about 1.0 and 2.1 gm./ cc. and an anisotropyratio from almost 1 up to about 150: 1. Surprisingly, the production ofsuch structures from compressed flakes produces a product of comparableproperties to that obtained by compressing vermicular graphite, yetrequires a volume decrease of only about 5:1 to 70:1 to obtain a compacthaving a density of about 2.0 gm./cc. as compared to a volume decreaseof from about 100:1 to 1000:1 for direct compression of the vermiculargraphite to a structure of the same density.

Formation of an integral, monolithic structure from compressed graphiteflakes usually requires a pressure of at least 300 p.s.i. to providegood bonding and a density of about 1.0. As the force of compressionincreases, the density of the final structure increases. Pressures aboveabout 50,000 p.s.i. continue to increase the density of the structurebut ever increasing proportions of force must be exerted to produceincremental increases in density. The application of a compressive forceof between about 300 p.s.i. and 50,000 p.s.i. are therefore usuallypreferred.

High density structures having improved mechanical properties may beprepared in accordance with this invention by blending the compressedflakes with a solid organic or inorganic bonding agent prior tocompression to form the cohered structure. Ordinarily, the organic orinorganic bonding agent is employed in the form of a fine powder, e.g.,from 100 to 325 mesh, in an amount of from about 2 to about 55 weightpercent and preferably from about 5 to about 45 weight percent bondingagent based on the total weight of the mixture. Bonding agents usefulherein include solid organic polymers, other organic compounds whichupon pyrolysis, yield a cementing char, inorganic glass-like bondingagents, and the like.

Examples of organic polymers suitable for use herein includepolyethylene, acrylic and methacrylic polymers, polystyrene, epoxides,phenol-formaldehydes, polyamides, polyesters, polyvinyl chlorides,polycarbonates, polytetrafluoroethylene, polyvinylidene fluoride,polyurethanes, copolymers and blends of the same, and the like. Thesebonding agents can be used along with any required catalyst orcrosslinker.

Examples of such other organic char yielding substances suitable for useherein include coal tar pitches, natural asphalts, phenol-formaldehyde,urea-formaldehyde, polyvinylidene chloride, and copolymers containingpolyvinylidene chloride, polymers of furfuryl, alcohol,polyacrylonitrile, sugar, saccharides and the like.

Examples of inorganic glass bonding agents suitable for use herein arevitreous materials which include, glassforming oxides such as boricoxide, silica, phosphorous pentoxide, germanium oxides, vanadiumpentoxide, and the like or other inorganic salts that can be obtained asglasses such as beryllium fluoride, and certain sulfates, chlorides andcarbonates. Especially useful in this invention are those glass-formerswhich will wet the graphite, such as B P 0 or V 0 Commercially availableglasses also are suitable as bonding agents. Typical examples of suchglasses are compositions containing as an ingredient various proportionsof two or more of the following oxide: silica, aluminum oxide, sodiumoxide, potassium oxide, magnesium oxide, cuprous oxide, barium oxide,lead oxide, or boric oxide.

Glass-forming oxides are defined as those oxides which are indispensableto the formation of oxide glasses. Those skilled in the art ofglass-making will readily recognize that the above named oxides aregenerally employed in combination With other materials to obtain glass.

The following examples are intended to further illustrate the inventionbut are not to be construed as limiting to its scope.

4 EXAMPLE 1 -Verrnicular graphite having a bulk density of about 0.004g-m./cc. was placed as individual particles and as small clumps ofparticles on a moving belt passing between two 6 inch diameter rollers,said belt having a clearance of about 0.001 inch. The particles werepassed between the rollers two to three times at a linear speed of about5 feet/minute and produced flattened, irregular, lacy flakes rangingfrom about one-half inch to several inches in length and havingthicknesses ranging from about 0.001 to about 0.005 inch. Such flakeshad a particle density of about 0.5 to about 1 gm./cc. as determined byimmersion in liquids. The bulk density of a lightly tamped volume ofsuch flakes was about 2.3 lbs./ ft. (0.037 gm./cc.) showing adensification of about 10- fold over the original vermicula-r graphite.

A 9 gram sample of such compressed flakes were placed in a 4%" x 1%" x4" mold and uniaxially compressed under a force of 17,000 p.s.i. alongthe 4' axis during a one minute compression cycle. The product of suchcompression was a flexible sheet having a thickness of 0.05 inch, adensity of 1.73 gm./-cc., an ultimate tensile strength of 1385 p.s.i.and a specific resistance (in the plane of the sheet) of 152tmicrohm-inches.

In comparison, a 9.2 gram charge of vermicular graphite 36 inches highin a mold measuring 4 /8" x 1%" was pressed to a height of 4 inches andthen compressed uniaxially in the same mold under a force of 17,000p.s.i. during a one minute compression cycle. The compressed product wasa sheet having a thickness of 0.065 inch, a density of 1.49 g-m./cc., anultimate tensile strength of 644 p.s.i. and a specific resistance (inthe plane of the sheet) of 220 microhm-inches.

EXAMPLE 2 In the same manner as Example 1, vermicular grap'hite having abulk density of about 0.25 lbs./ft. (0.004 gm./cc.) was placed asindividual particles and as small clumps of particles .on a moving beltpassing between rollers. The compressed product was a collection offlattened, irregular, lacy flakes having a thickness to length ratio offrom about 60:1 to about 500:1 and having a bulk density (if lightlytamped) of about 2.2 lbs/ft. (0.036 gm./cc.). This material could beeasily tamped to a density of 6 lbs/ft. and .refluffed to its originaldensity and characteristics by stirring.

A mold 4 inches tall was filled with such flakes and a pressure of17,000 p.s.i. was applied thereto during a compression cycle of 30seconds by single stroke of a piston. The resulting compressed structurehad a density of 1.63 gm./cc. and a thickness of about 0.053 inch.

In comparison, a mold having the same cross sec-tion and 24 inches highwas filled with verimicular graphite having a density of 0.004 gm./ cc.and compressed to a density of 0.085 g-m./ cc. This low density compactwas then inserted into the 4 inch mold employed above and com-pressedunder a force of 17,000 p.s.i. applied by the single stroke of a pistonduring a 30 second compression cycle. The resulting compressed structurewas spongy, had a thickness of 0.076 inch and had a bulk density of 1.12gm./cc.

EXAMPLE 3 In the same manner as Example 1, particles and clumps ofparticles of vermicular graphite having a bulk density of 0.004 gm./cc.was passed between rollers. The resulting irregular flakes had a bulkdensity (lightly tamped) of about 0.036 gm./'cc. Upon uniaxialcompression of a random mass of such flakes, a compressed structure wasproduced which had a density of 1.77 -gm./cc., a specific resistance inthe plane perpendicular to the axis of compression of microhm-inches, aspecific resistance in the plane parallel to the compression axis of19,500 microhm-inches and a tensile strength of 1075 p.s.i. in thedirection perpendicular to that of compression.

In the same manner as Example 1, compressed flakes having a density ofabout 0.036 gm./cc. were produced by compressing particles of vermiculargraphite. A mass 3. The process of claim 1 wherein the compressedgraphite flakes are formed into a graphite article by compressing undera force of at least 300 psi.

4. The process of claim 1 wherein the compressed of Such particles wereplaced in a rubber bag and the 5 graphite flakes are formed into agraphite article by bag was evacuated to remove the air therefrom. Afterevacuation, the bag was sealed and i-sostatically compressed under aforce of 50,000 psi. The product was a cohered monolithic graphitestructure having a density of 1.94 gm./cc.

Various modifications can be made in the present invention withoutdeparting from the spirit or scope thereof for it is understood that welimit ourselves only as defined in the appended claims.

We claim:

1. A process for the preparation of compressed graphite articles whichcomprises compressing particles of a low density vermicular graphite toform individual compressed graphite flakes having a particle density ofbetween about 0. 25 and about 1.5 grn./cc. and subsequently compressinga mass of such compressed graphite flakes into a cohered, monolithicarticle having a density of between about 1.0 and 2.1 gmJcc.

2. The process of claim 1 wherein the vermicular graphite particles arecompressed by passing such particles between rollers.

compressing under a force of between about 300 p.s.i. and about 5000psi.

5. The process of claim 4 wherein the compressive force is applieduniaxially.

6. The process of claim 4 wherein the compressive force is appliedmultiaxially.

References Cited UNITED STATES PATENTS 1,137,373 4/1915 Aylsworth264l-09 2,127,994 8/1938 Davis 264-109 2,888,7 15 6/1959 Frank 2642,997,744 8/ 1961 Stoddard 23-2O9.1 3,054,662 9/1962 Gessler 2-3-20913,107,973 10/ 1963 Bickerdike 23-2091 FOREIGN PATENTS 231,307 9/ 1958Australia.

NORMAN YUDKOFF, Primary Examiner.

US. Cl. X.R. 23-209.1, 293, 314; 264-109

